Most underwater "plants" are algae, too primitive to produce flowers. Their life cycles are complicated and the fecundation happens in special short lived individuals called gametophytes that are generated from spores and produce gametes that fuse into other spores. They don't produce pollen, so can't be pollinated.
To speak crudely, pollen is like a vase that hosts inside a crew of "spermatozoan like" gametes and then injects them directly in the right place. Algae just release the gametes without the "mothership". Some have "smart" gametes that act basically like tiny animals. This was a headache for a lot of time until a new kingdom was created for "neither exactly a plant nor an animal".
The upper aquatic plants normally just raise the flowers over the surface to use the old reliable insects (Lotus). Only a few remain totally submerged at all times (Posidonia) and those probably use crustaceans and sea currents as pollinators.
I couldn't find any evidence of Posidonia using crustaceans for pollination, so it seems they just use the sea current, similar to wind pollination over water. However, underwater animal pollination does exist in some seagrasses, namely by help of marine worms:
> The Posidonia herbarium has a very high life expectancy. According to some scientists, this species can live several centuries. Some specimens aged over 80,000 years have been found in the Balearic Islands.
To speak crudely, pollen is like a vase that hosts inside a crew of "spermatozoan like" gametes and then injects them directly in the right place. Algae just release the gametes without the "mothership". Some have "smart" gametes that act basically like tiny animals. This was a headache for a lot of time until a new kingdom was created for "neither exactly a plant nor an animal".
The upper aquatic plants normally just raise the flowers over the surface to use the old reliable insects (Lotus). Only a few remain totally submerged at all times (Posidonia) and those probably use crustaceans and sea currents as pollinators.